1. Field of the Invention
The present invention relates to a multi-hop relay Broadband Wireless Access (BWA) system. More particularly, the present invention relates to an apparatus and method for managing a packet which is transmitted to a Mobile Station (MS) that performs a Relay Station (RS) handover in a multi-hop relay BWA system.
2. Description of the Related Art
In a next generation communication system, also known as the 4th Generation (4G) communication system, extensive research is being conducted to provide a Quality of Service (QoS) with a data transfer speed of about 100 Mbps. More particularly, in a Broadband Wireless Access (BWA) system, such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system, research is being conducted into a communication system that supports a high speed service at the same time as providing mobility and ensuring a QoS. An example of such a communication system is an Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system, which standard is hereby incorporated by reference.
The IEEE 802.16 communication system employs an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme so as to allow a physical channel of the BWA system to support a broadband network. In order to ensure mobility of a Mobile Station (MS) and flexibility of implementing a wireless network in the IEEE 802.16 communication system, extensive research is also being conducted to effectively provide services in a wireless environment where traffic distribution and call demands rapidly change. For example, a communication system is taken into account to cope with these demands, in which a Relay Station (RS) is used to transfer data based on a multi-hop relay scheme.
In the BWA system, the RS is used to increase the coverage area or improve throughput. That is, throughput between a Base Station (BS) and an MS can be improved when the RS is located in a specific area having a poor channel condition. In addition, the BS can communicate with an MS that exists outside a coverage area of the BS when the RS is located near a cell boundary.
In general, since RSs are located within a cell of a BS, RS-to-RS handover occurs more frequently than BS-to-BS handover That is, handover occurs more frequently when an RS is used, which leads to erroneous conditions such as a packet loss and a service delay.
In a situation where an RS is not used (i.e., communication is achieved only between a BS and an MS), bicasting is used to avoid the erroneous conditions caused by a handover. In the bicasting process, as shown in FIG. 1, a data packet to be transmitted to an MS is transmitted to two BSs, where the MS is located in a boundary area of the two BSs, that is, handover is expected to be performed. Therefore, after the MS performs handover, a target BS does not have to receive from a serving BS a data packet which is buffered to be transmitted to the MS. However, since the bicasting corresponds to multicasting in a wired network, it is difficult to apply the bicasting to a wireless network environment using, for example, the RS and the BSs.
As described above, when an RS is used in a multi-hop relay BWA system, a system performance deteriorates due to a frequent handover. Although the bicasting has been proposed to avoid the system performance deterioration, it is not appropriate to apply the bicasting in a wireless network environment using the RS. Therefore, there is a need for a method for continuously providing a service to an MS that performs inter-RS handover.